Add support for scanning QR codes during verification, with Rust crypto (#3565)

* Offer `m.qr_code.scan.v1` verification method by default

Normally, the application specifies the supported verification methods when
creating the MatrixClient (and matrix-react-sdk does so). If the application
leaves it unset, then the idea is that the js-sdk offers all known verification
methods.

However, by default, the rust-sdk doesn't specify `m.qr_code.scan.v1`. So
basically, we need to set our own list of supported methods, rather than
relying on the rust-sdk's defaults.

* Factor out base class from `RustSASVerifier`

* Implement QR code scanning

* Update src/rust-crypto/verification.ts

src/crypto-api/verification.ts

@@ -136,12 +136,27 @@ export interface VerificationRequest
     /**
      * Send an `m.key.verification.start` event to start verification via a particular method.
      *
+     * This is normally used when starting a verification via emojis (ie, `method` is set to `m.sas.v1`).
+     *
      * @param method - the name of the verification method to use.
      *
      * @returns The verifier which will do the actual verification.
      */
     startVerification(method: string): Promise<Verifier>;
 
+    /**
+     * Start a QR code verification by providing a scanned QR code for this verification flow.
+     *
+     * Validates the QR code, and if it is ok, sends an `m.key.verification.start` event with `method` set to
+     * `m.reciprocate.v1`, to tell the other side the scan was successful.
+     *
+     * See also {@link VerificationRequest#startVerification} which can be used to start other verification methods.
+     *
+     * @param qrCodeData - the decoded QR code.
+     * @returns A verifier; call `.verify()` on it to wait for the other side to complete the verification flow.
+     */
+    scanQRCode(qrCodeData: Uint8Array): Promise<Verifier>;
+
     /**
      * The verifier which is doing the actual verification, once the method has been established.
      * Only defined when the `phase` is Started.